There is disclosed herein, in various embodiments, an imaging member for reducing charge deficient spots. In the embodiments, the imaging member has a charge generating layer and a charge transport layer with multiple concentrations or regions of particular small molecule charge transport materials. The region or layer closest in proximity to the charge generating layer comprises a lower concentration of charge transport material than a layer spaced from the charge generating layer. Such arrangements and compositions reduce charge injection from the charge generating layer into the charge transport layer thereby reducing the formation of charge deficient spots.
A typical electrophotographic imaging member is imaged by uniformly depositing an electrostatic charge on an imaging surface of the electrophotographic imaging member and then exposing the imaging member to a pattern of activating electromagnetic radiation, such as light, which selectively dissipates the charge in the illuminated areas of the imaging member while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking toner particles on the imaging member surface. The resulting visible toner image can then be transferred to a suitable receiving member, such as paper.
A number of current electrophotographic imaging members are multilayered photoreceptors that, in a negative charging system, comprise a substrate support, an electrically conductive layer, an optional charge blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer, and optional protective or overcoating layer(s). The multilayered photoreceptors can take several forms, for example, flexible belts, rigid drums, flexible scrolls, and the like. Flexible photoreceptor belts may either be seamed or seamless belts. An anti-curl layer may be employed on the back side of the flexible substrate support, the side opposite to the electrically active layers, to achieve a desired photoreceptor belt flatness.
Although excellent toner images may be obtained with multilayered belt photoreceptors, a delicate balance in charging image and bias potentials, and characteristics of toner/developer must be maintained. This places additional constraints on photoreceptor manufacturing, and thus, on the manufacturing yield. Localized microdefect sites, varying in size of from about 5 to about 200 microns, can sometimes occur in manufacture, which appear as print defects (microdefects) in the final imaged copy. In charged area development, where the charged areas are printed as dark areas, the sites print out as white spots. These microdefects are called microwhite spots. In discharged area development systems, where the exposed area (discharged area) is printed as dark areas, these sites print out as dark spots on a white background. All of these microdefects, which exhibit inordinately large dark decay, are called charge deficient spots (CDS). Since the microdefect sites are fixed in the photoreceptor, the spots are registered from one cycle of belt revolution to next. Charge deficient spots have been a serious problem for a very long time in many organic photoreceptors, such as multi-layered benzimidazole perylene photoreceptors where the perylene pigment is dispersed in a matrix of a bisphenol Z type polycarbonate film forming binder.
Whether these localized microdefect or charge deficient spot sites will show up as print defects in the final document depends, to some degree, on the development system utilized and, thus, on the machine design selected. For example, some of the variables governing the final print quality include the surface potential of photoreceptor, the image potential of the photoreceptor, photoreceptor to development roller spacing, toner characteristics (such as size, charge, and the like), the bias applied to the development rollers and the like. The image potential depends on the light level selected for exposure. The defect sites are discharged, however, by the dark discharge rather than by the light. The copy quality from generation to generation is maintained in a machine by continuously adjusting some of the parameters with cycling. Thus, defect levels may also change with cycling.
Techniques have been developed for the detection of CDS's. These have largely involved destructive testing, although some contactless methods have been developed. Additionally, multilayer imaging members have been developed to block charge injection from the substrate which can give rise to CDS's.
Furthermore, despite the various known photoreceptor designs, there remains a need in the art for methods of reducing the occurrence of charge deficient spots in the first instance and/or to mitigate their effect in the photoreceptor during use. If the occurrence of charge deficient spots can be reduced or eliminated, or if their effect in the photoreceptor during use can be mitigated, then resultant print quality using the photoreceptors will increase and photoreceptor production yield should also increase. Longer photoreceptor useful life if particularly desired, for example, because it makes image development and machine service more cost effective, and provides increased customer satisfaction.